This invention is directed to a method and apparatus for connecting a ground power unit to an existing aircraft power bus under what is known as a no-break power transfer (NBPT) operation.
Modern day commercial aircraft employing sophisticated on-board electronic equipment require that the power transfer between the aircraft power generating systems, such as engine driven generators or an auxiliary power unit (APU), and a ground power unit (GPU) be accomplished without any interruption or break whatever. In these aircraft, after a ground power unit is attached to the aircraft, the pilot activates a power transfer switch and power transfer will take place with both systems being on line simultaneously for a brief interval, Until that connection is actually made, however, the ground power unit is isolated from the aircraft power system and cannot detect any characteristic of the aircraft power bus, and in particular, it cannot detect the relative phase between the two systems; therefore no adjustment of the phase angle of the GPU can be made beforehand.
On the other hand, the electrical system of some aircraft can monitor both power systems and will accomplish power transfer only when the phase difference is 90.degree. or less, depending on the system. In other aircraft, the connection is made without regard to the relative phase angles. Connecting the two systems with large phase angle differences could cause one or both of the systems to disconnect or fail.
Since there is no pre-connection detection of the relative phase of the two systems at the GPU side of the system, the GPU must be capable of adjusting quickly when the connection is actually made, to remain on line throughout the connection process, and to provide the proper voltage and frequency to the aircraft while at the same time preventing any failure or damage to either power system.
Accordingly, there is a need for a ground power unit that can interface with an aircraft electrical system to provide an uninterrupted source of power of the proper voltage and frequency even when the two systems are not in phase at the time they are initially interconnected.